1. Field of the Invention
This invention relates generally to a recording or printing device utilizing a jet of liquid ink droplets to form a printed image which droplets are selectively charged as they move toward the paper or other medium for receiving the droplets.
2. Description of the Prior Art
Ink jet type printing devices are well known wherein a stream of ink droplets emerge from a nozzle print head as a jet of ink, and then the droplets are suitably charged so they may be deflected by horizontal deflection plates during their travel toward the paper for receiving them in the desired configuration. Thus, the ink droplets upon impinging on the paper surface will form sets of dots which represent letters, numbers, or desired symbols.
Since the amount of charge on each ink drop is controlled individually, a drop can be deflected by a desired amount in a vertical direction. As the drops are deflected vertically, the printhead is moved horizontally at a constant speed. The droplets which are not required in forming a particular character are left uncharged and are undeflected. These droplets are intercepted by a gutter and are recycled to an ink reservoir.
Since the droplets are ejected serially from a moving source, it is obvious that each succeeding droplet will have moved some finite distance in the direction of motion relative to the preceding droplet. The amount of displacement is proportional to the printhead velocity and the frequency with which the droplets are ejected.
In printing a character, the droplets are deflected from bottom-to-top during a scan so that by the time the top most droplets reach the printing surface, the dots they produce have been displaced horizontally relative to the bottom dots previously printed. Unless some compensation is performed, the dots printed during each scan would "lean" in the direction of printhead motion. Furthermore, in the case of bidirectional printing, the characters would "lean" in alternate directions depending upon the direction of motion of the printhead when the character was formed, thus producing unacceptable print quality.
One previous method of overcoming this problem was to incline fixed deflection plates at a fixed angle, determined by the scan repetition rate and velocity of the printhead carriage, and print in only one direction. This, of course, limited the printer to one speed and undirectional printing only.
However, to accomplish bidirectional printing using a fixed deflection plate angle, the characters could be scanned from bottom-to-top when the printhead moved in one direction; and scanned from top-to-bottom when the printhead moved in the opposite direction. This provides bidirectional printing at one speed, but requires a dual scanning mode and doubles the memory space required for dual compensation storage. For example, see U.S. Pat. No. 4,075,636 and U.S. Pat No. 4,321,609.
The problem noted above can be partially solved by mechanically changing the deflection plate angle to automatically compensate for horizontal motion of the printhead regardless of the direction of printing or speed. This allows the development of a more versatile printer having the capability of printing bidirectionally.
Several known devices provide structure for partially solving the above problems. For example, U.S. Pat. No. 4,219,823 to Fathergill et al, mounts deflection electrodes 14, 15 upon a ring assembly 21 which is rotatable by electric solenoids so that the deflection electrodes will be tilted in one direction or the other. Another known prior art device to Denney et al, U.S. Pat. No. 4,246,589, mounts deflection electrodes 14, 15 upon a rocker member 22 which will tilt the deflection electrodes in one direction or the other to compensate for the skewing of the ink jet droplets as the carriage moves laterally of the print medium 16. While both of the devices of these patents will compensate for ink jet skewing for a particular speed of carriage traverse, neither provides for different speeds of carriage traverse by arranging for different degrees of tilt of the deflection electrodes to correspond to the different traverse speeds.
However, none of the known prior art devices offer the new and novel features of the present invention.